This afternoon the UN Security Council met in an emergency session. That's amid reports of new Russian incursions into Ukraine opening another front in the conflict there. Russia keeps denying that its forces are fighting inside Ukraine. But the U.S. and NATO say this covert war is not covert anymore. NPR's Michele Kelemen reports on the mounting evidence of Russia's involvement and what the West plans to do about it.

MICHELE KELEMEN, BYLINE: As NATO released more satellite imagery of Russian tanks and troops inside Ukraine, that country's prime minister, Arseniy Yatsenyuk, appealed to the world for help.

PRIME MINISTER ARSENIY YATSENYUK: Ukrainian forces are capable to tackle and to cope with the Russian-led guerrillas. But this is quite difficult for us to fight with Russia and its army.

KELEMEN: Yatsenyuk says the U.S. and its partners can start by freezing all Russian assets until Russian President Vladimir Putin withdraws his troops. Damon Wilson of the Atlantic Council has another idea for the U.S. and NATO - share intelligence with Ukrainian forces and beef up the country's defenses. He dismisses those who have argued such moves would only serve to escalate the conflict.

DARREN WILSON: The realities that we're seeing is that it's actually escalatory not to support the Ukrainians with these types of capabilities. Because it'll serve to embolden Putin to think not only can he get away with it, but he can double-down on it.

KELEMEN: Wilson expects the crisis in Ukraine to dominate next week's NATO summit. And he thinks the alliance will move to shore up its members near Russia.

WILSON: The problem is as the alliance has strengthened its solidarity with its own allies, it's just made it all the more clear that those that are on the other side of the line are really vulnerable to Russia or vulnerable to Russia's fear of influence, to their pressure and to these military tactics.

KELEMEN: This is not just Ukraine's war, Wilson says, but a challenge to European security. At the State Department, Spokesperson Jen Psaki says the U.S. is considering a range of options but she said for now is focusing on non-lethal aid to Ukraine. Psaki says there is a diplomatic offering for the Russian president. Though for now, she acknowledges, he hasn't taken it.

JEN PSAKI: What we're seeing - not just over the last couple of days, but certainly weeks and even months - is a pattern of escalating aggression in Ukraine from the Russians and Russian-backed separatist.

JEFFREY FELTMAN: We cannot ignore the deeply alarming reports of Russian military involvement in this new wave of escalation. If confirmed, it would constitute a direct contravention of international law and of the UN charter.

KELEMEN: Russia's Ambassador, Vitaly Churkin, says there may be some Russian volunteers in Ukraine. But he says that's not the problem.

VITALY CHURKIN: (Through translator) The current escalation in the southeast of the Ukraine is a direct consequence of the reckless policy of Kiev, which is conducting war against its own people.

KELEMEN: But the U.S. Ambassador to the UN, Samantha Power, says the mask is coming off.

SAMANTHA POWER: Serious negotiations are needed - urgently needed - but Russia has to stop lying and has to stop fueling this conflict.

KELEMEN: The U.S. says it will continue to work in lockstep with partners in Europe on this. Wilson of the Atlantic Council says Washington is right to coordinate with European partners like Germany to put pressure on Putin to change course. But he argues at this point the U.S. needs to be out in front.

WILSON: If there's a perception in Moscow that we're just outsourcing this crisis to Berlin, Moscow's going to conclude there's much farther they can go and get away with on the ground.

KELEMEN: German Chancellor Angela Merkel says she and her European colleagues will discuss more sanctions when they meet at an EU summit on Saturday. Michele Kelemen, NPR News, Washington. Transcript provided by NPR, Copyright NPR.